OSPF protocol, a widely used routing protocol, employs a link-state arithmetic. In brief, each router tests its link state to each of its neighbors and forms LSAs. Then each router calculates the routing information according to all received LSAs that propagate throughout the autonomous system network.
In OSPF protocol, type 5 LSA is used to represent external routing. With the great development of network technology, the number of type 5 LSAs will be exceedingly increased, possibly reaching several hundred thousand. The Internet Engineering Task Force (IETF) Request For Comments (RFC) 2328 standard defines that all the LSAs originated by routers are flushed at every interval, say 1,800 seconds, i.e., each new version of every LSA is sent out at the interval. In this instance, a LSA will be regarded by other routers that it has been deleted if it is not flushed within 3,600 seconds.
This is a simple fixed interval processing mode. In this mode, after originating a LSA, a network device will send a new version of it after 1,800 seconds, i.e., flush for once. This mode has a disadvantage that the flushing processing will occupy a great lot of network bandwidth resources if the number of type 5 LSAs is huge enough. This will decrease the network traffic and impact on the processing of other network devices.
Suppose a network device originates several hundred thousand type 5 LSAs at one time, then all the LSAs will be flushed simultaneously in 1,800 seconds. At the moment, several ten thousand packets appeared in the network. It is very possible that the network cannot be loaded with such intense traffic, then parts of the packets are lost and will be multiply re-transmitted. Moreover, the flushed LSAs will propagate to all network devices in this network, which impacts on other devices. Some network devices, especially those with low processing capabilities, may suffer from block, no response and services interruption etc.